Highly Branched Waxy Potato Starch-Based Polyelectrolyte: Controlled Synthesis and the Influence of Chain Composition on Solution Rheology

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University of Groningen

Highly Branched Waxy Potato Starch-Based Polyelectrolyte

Fan, Yifei; Bose, Ranjita K.; Picchioni, Francesco

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Industrial and Engineering Chemistry Research DOI:

10.1021/acs.iecr.9b06893

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Fan, Y., Bose, R. K., & Picchioni, F. (2020). Highly Branched Waxy Potato Starch-Based Polyelectrolyte: Controlled Synthesis and the Influence of Chain Composition on Solution Rheology. Industrial and Engineering Chemistry Research, 59(23), 10847-10856. https://doi.org/10.1021/acs.iecr.9b06893

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Highly Branched Waxy Potato Starch-Based Polyelectrolyte:

Controlled Synthesis and the In

ﬂuence of Chain Composition on

Solution Rheology

Yifei Fan, Ranjita K. Bose, and Francesco Picchioni

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sı Supporting Information

ABSTRACT: In the present work, a series of highly branched random copolymers of acrylamide (AM), sodium 2-acrylamido-2-methyl-1-propanesulfonate (SAMPS), and N-isopropylacrylamide (NIPAM) were prepared by using water-soluble waxy potato starch-based macroinitiator via aqueous Cu0-mediated living radical polymerization (25°C). The AM/SAMPS/NIPAM ratio was varied to investigate the inﬂuence of chain composition on the aqueous rheological properties of the prepared copolymers. Rheological results indicated an optimum SAMPS intake (25 mol %) for the balanced performance of viscosity and salt resistance in saline water. The intake of NIPAM units (e.g., 25 mol %), contrary to what was expected, undermines the thickening ability of the copolymers in saline water because of hydrophobic association. In high salinity solution, thermo-thickening behavior can be observed at low shear

rates (γ ≤ 3 s−1) because of the screening effect of salt on the negatively charged SAMPS units. At the high shear rate, the thermo-thickening behavior disappears because of the disruption of the NIPAM aggregates. These results pave the way toward the use of the prepared polymers as rheology modifiers in a variety of possible formulations for different applications, in particular in enhanced oil recovery.

1. INTRODUCTION

Enhanced oil recovery (EOR) is a widely used technique to promote oil production because of the increasing demand for oil and the relatively large portion (∼40% of original oil) of unexploited residual oil.1 Polymer flooding is one common EOR method entailing the use of a polymer as a thickener to reduce the mobility difference between water and oil and thus increase the sweep efficiency of water on the porous structure, where crude oil is deposited. Partially hydrolyzed polyacryla-mide (HPAM) has been widely used in recent decades for polymerflooding because of its low cost.2,3However, HPAM tends to hydrolyze and precipitate out in the reservoirs, especially in those with high temperatures (up to 130°C) and salinity (up to 200,000 ppm, in particular when divalent cations such as Ca2+ and Mg2+ are present).4−6 This is detrimental to the performance of polymer flooding and also harmful to the reservoir. In this case, carefully designed polymers, which can withstand the hostile (saline) environ-ment, are needed for sustainable exploration. Another problem in the oilfield exploitation is the management of produced water which is a byproduct with a large volume. This kind of water usually contains a variety of organic and inorganic pollutants such as hydrocarbons, heavy metals, chemicals during drilling, and in most cases also has high salinity.7

Reinjection of produced water for EOR is one attractive option for the sake of the environment (fragile ecosystems such as offshore and Arctic areas) and reservoir protection (the use of a different water source might result in the significant formation of precipitated salts, thus damaging the reservoir exploration).8 The utilization of saline-resistant polymers may be helpful to achieve this purpose.

The salt-resistance of polymers, like many other properties, is also affected by both their composition and structure. For example, our previous studies revealed that branched structures can result in higher viscosity and better viscoelastic properties than linear ones because of their more stable hydrodynamic volume inherent to the steric hindrance in the structure.9−11As a matter of fact, copolymers based on the homogeneous grafting of highly branched waxy potato starch did display both thermo-thickening behavior and stable viscosity profile in different saline solutions.12−15However, efforts are still needed

Received: December 16, 2019 Revised: May 11, 2020 Accepted: May 13, 2020 Published: May 13, 2020 Article pubs.acs.org/IECR

https://dx.doi.org/10.1021/acs.iecr.9b06893 Ind. Eng. Chem. Res. 2020, 59, 10847−10856 This is an open access article published under a Creative Commons Non-Commercial No

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to improve the solubility (dissolve faster) and viscosity of copolymers compared with that of commercial HPAM products. One option is the incorporation of salt-resistant polyelectrolyte segments in the highly branched structure. Poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salts) (PAMPS), as an anionic polyelectrolyte bearing negative charges in polymer chains such as HPAM, is reported to be more stable at high temperature in saline water.16,17Moreover, PAMPS polymers are now also recognized as potential materials for bioengineering,18 forward osmosis,19,20 and catalyst support for energy and environment applications.21 Thus, the synthesis of PAMPS and its copolymers has been an interesting research topic in recent years.22−25

In the case of grafting PAMPS (co)polymers onto starch, generally, the reaction is carried out in water by free radical polymerization with ceric salts as the initiator.23,26As reported, by using controlled polymerization, such as transition-metal [mainly Cu(I)]-mediated atom transfer radical polymerization (ATRP), Cu0_{-mediated aqueous living radical polymerization}

(Cu0-mediated LRP), and reversible addition−fragmentation chain transfer (RAFT), the formation of homopolymer, typical for conventional free radical polymerization, can be avoided and tailor-made properties can be achieved.12,22,23 This is favorable for the investigation of the inﬂuence of chain length and structure on polymer properties. Compared with RAFT, which usually should be carried out above 50°C, and normal ATRP, Cu0_{-mediated LRP could be achieved at room}

temperature or even lower (e.g., 0 °C ) while maintaining a much higher polymerization rate in aqueous solution.27 However, reports on the synthesis of PAMPS and its copolymers by Cu0_{-mediated LRP are still rare compared}

with RAFT and“classical” ATRP.24,28−30

In the present study, the controllability of Cu0_-mediated

LRP on the synthesis of poly(sodium 2-acrylamido-2-methyl-1-propanesulfonate) (PSAMPS) in water was veriﬁed. Then, a water-soluble macroinitiator was synthesized by homogeneous esteriﬁcation of waxy potato starch with 2-bromopropionyl bromide (BpB) in N,N-dimethylacetamide (DMAc).31,32 Based on this, a series of branched random copolymers of acrylamide (AM), sodium 2-acrylamido-2-methyl-1-propane-sulfonate (SAMPS), and N-isopropylacrylamide (NIPAM) were prepared by aqueous Cu0-mediated living radical polymerization (Cu0_{-mediated LRP) at room temperature}

(illustration seeFigure S1). The macroinitiator and polymers were characterized by nuclear magnetic resonance (NMR) and Fourier transform infrared (FT-IR). For potential applications such as EOR, the ratios of the three monomers were varied to study the inﬂuence of chain composition on the rheological properties of polymer solutions and their response to temperature and saline water.

2. MATERIALS AND METHODS

2.1. Materials. Waxy potato starch (>95% amylopectin) was kindly donated by Avebe (the Netherlands) and dried under vacuum at 60°C for 48 h before use. Lithium chloride from Sigma-Aldrich was dried under vacuum at 80°C for 24 h before use. Copper powder (<75μm) from Sigma-Aldrich was stored under a N₂ atmosphere. Anhydrous DMAc, 2-bromopropionic acid, BpB, formaldehyde solution (37%), formic acid (>95%), and AM were purchased from Sigma-Aldrich and used as received. 2-Acrylamido-2-methyl-1-propanesulfonic acid (AMPS) was purchased from Aldrich and neutralized with sodium hydroxide (from

Sigma-Aldrich) in Milli-Q solution which was then poured into tenfold acetone (cooled with an ice bath) to obtain the solid SAMPS. Tris(2-aminoethyl)amine (Tren) was purchased from TCI and used as received for the synthesis of tris[2-(dimethylamino)ethyl]amine(Me₆Tren) following the proce-dures reported.33NIPAM (stabilized with MEHQ) from TCI was recrystallized from acetone to remove the inhibitor.

2.2. Characterization. The composition of the synthesized copolymer was determined by an Elementar vario MICRO cube CHNS elemental analyzer. For a starch-based grafted copolymer without NIPAM intake, samples were taken after Soxhlet extraction as the trace solvent (ethanol) impurities have no inﬂuence on the result. For the NIPAM containing copolymers, samples from the Soxhlet extraction were dissolved in Milli-Q water and rotovap was used to remove the trace solvent. The obtained samples were freeze-dried for one day before the test. The ratio of monomer units can be obtained according to the following equation

S N (SAMPS) 14 % 32 % γ = × × (1) C N S N (AM) 1 3 6 14 % 12 % 14 % 32 % i k jjj y_{zzz γ = × − × × + × × (2)

where γ(SAMPS) and γ(AM) stand for the mole ratio of SAMPS and AM in the copolymer, respectively, C %, N %, and S % is the mass ratio of carbon, nitrogen, and sulfur in the copolymer, respectively (seeTable S1).

A Varian Mercury Plus 400 MHz spectrometer was employed for the recording of NMR spectra with deuterated water from Sigma-Aldrich as the solvent. FT-IR spectra were obtained via an IRTracer-100 SHIMADZU FT-IR spectropho-tometer equipped with attenuated total reﬂection accessories and LabSolutions IR software. An Agilent 1200 aqueous gel permeation chromatography (GPC) system equipped with a diﬀerential refractive index detector and Polymer Standard Service (PSS) column set (PSS SUPREMA 100, 1000, 3000 Å) was used for the characterization of molecular weight and distributions. The mobile phase used was 0.05 M NaNO3

aqueous solution with a flow rate of 1 mL/min. The system temperature was regulated to 40 °C. Polyethylene oxide standards (ReadyCal, M_p 200−1,200,000) from Fluka were used for calibration. Samples were filtered through a Teflon membrane with 0.22 μm pore size before injection. The experimental molecular weight and polydispersity index (PDI) of synthesized polymers were determined via Agilent GPC/size exclusion chromatography software.

A HAAKE Mars III (Thermo Scientiﬁc) rheometer equipped with a cone-and-plate geometry (diameter 60 mm, angle 2°) was employed for the characterization of rheological properties of polymer solutions. The viscosity of polymer solution was measured as a function of shear rate (0.1 to 1750 s−1, T = 20 °C), salt concentration (5000−100,000 ppm of NaCl, T = 20°C, shear rate 10 s−1), and temperature (10 to 95 °C, shear rate 1, 3 and 10 s−1_{), respectively.}

Martin equation was used to determine the intrinsic viscosity of polymer solutions34 c e c red sp _M η η η = = [ ] κ [ ]η (3) whereηredis the reduced viscosity,ηspis the speciﬁc viscosity, c

is the polymer concentration, [η] is the intrinsic viscosity, and kMis a constant that depends on the polymer−solvent system.

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The Carreau-Yasuda model was used for the determination of relaxation time (λ)9,35,36 1 ( ) n 0 1/ η η η η λ γ − − = [ + · ] α α ∞ ∞ − (4) whereη is the viscosity, η0is the zero shear rate viscosity,η∞is

the viscosity at the inﬁnite shear rate, 1/λ is the critical shear rate for the onset of shear thinning, n−1 is the power-law index, and α represents the transition region between η0 and

the power-law region.

2.3. Synthesis of Starch-Based Macroinitiator. Waxy potato starch (1.30 g, 8 mmol) and lithium chloride (0.51 g, 12 mmol) were added to a 100 mL three-neckedflask (dried at 100°C for 12 h before use) equipped with a mechanical stirrer. The oxygen and residual water in the system were removed via three evacuate−refill cycles under heat with N₂. Anhydrous DMAc (50 mL) was injected to theflask and the mixture was stirred at 130°C for about 1 h under a N₂atmosphere then cooled down to obtain a transparent solution. BpB (0.21 mL, 2 mmol) was added dropwise within 15 min to the starch solution cooled with an ice bath under the protection of N2.

The reaction mixture was then stirred for 3 h at room temperature. The crude product was precipitated out with tenfold acetone andfiltered, washed then dried under vacuum at 45 °C for 24 h. The obtained white powder was further purified by Soxhlet extraction with acetone as the solvent for 24 h and thefinal yield was 87%.

2.4. Synthesis of Starch-Based Copolymers by Aqueous Cu0_{-Mediated LRP. 2.4.1. Typical Polymerization}

Protocol. H2O (50 mL), starch-based macroinitiator (StBr)

(24.3 mg, 0.02 mmol), a mixture of AM and SAMPS (120 mmol in total), together with Me6TREN (12μL, 0.04 mmol)

were charged to a 100 mL three-neck round-bottomﬂask then deoxygenated by three freeze pump thaw cycles. Cu powder (2.6 mg, 0.04 mmol) was added subsequently with rapid stirring under the protection of nitrogen. The reaction mixture was stirred at room temperature for 15 min. The copolymer was separated out via freeze-drying the reaction solution and followed by Soxhlet extraction with ethanol as the solvent for 48 h. The pure product was then vacuum dried at 65°C for 48 h. For the purpose of short and clear, starch-g-Poly(AM-co-NIPAM-co-SAMPS) copolymers were named as ANS0000 and ANS1510 and so forth, in which the four digits stand for the ratio of NIPAM (N) and SAMPS (S) in the feeding solution, respectively (details seeTable 2).

2.5. Cleaving of Graft Polymer Chains from the Starch Backbone. Starch-based copolymer (0.10 g) was dissolved in 10 mL Milli-Q water in a round-bottom ﬂask together with 0.10 mL concentrated hydrochloric acid. The reaction mixture was stirred and reﬂuxed at 100 °C for 3 h. The resulting free (co)polymer was precipitated out and washed with acetone three times then dried under vacuum at 60°C for 24 h.

3. RESULTS AND DISCUSSION

The successful synthesis of a water-soluble waxy potato StBr was proved by FT-IR and NMR (1_H-NMR, 13_{C-NMR, and}

gHSQC). Details can be found in theSupporting Information (Figures S2 and S3).

Compared with the synthesis of low degree of polymer-ization (DP) polymer, normally a higher catalyst/initiator ratio is needed to obtain a high DP polymer with controlled Cu0

-mediated LRP. This is in agreement with the literature.27As shown in Table 1, linear PSAMPS (co)polymers were

synthesized with diﬀerent DP (at diﬀerent catalyst/initiator ratio) to verify the control on the Cu0_{-mediated LRP}

polymerization. PDI values in Table 1 suggest a relatively narrow distribution of (co)polymers, which constitutes a proof of controlled polymerization.

For homopolymers, the kinetic of polymerization was also monitored by1H-NMR and the kinetic plots (Figure 1) were thus obtained according toeq 5.37

M M k R k t

Ln( ₀/ _t)= _p( / )_i _t 1/2· ₍₅₎

where M0 and Mt are the monomer concentration at the

beginning of polymerization and at time t, respectively, k_p indicates the kinetic propagation constant, Rithe initiation rate

while k_i the termination rate constant. Clearly, the linear relationship between ln(M0/Mt) and t in Figure 1 reveals a

reasonably controlled polymerization, which is in line with the PDI values from GPC (Table 1). Besides, the conversion and kinetic curves also indicate an induction period in polymer-ization which is attributed to the absence of soluble copper species (CuI _{and Cu}II _{species) at the beginning of}

polymer-ization.38 Brieﬂy, the CuI is also an eﬃcient activator to accelerate the polymerization progress but its accumulation in the system via initial Cu0activation and CuII disproportiona-tion afterward (Scheme 1) needs time which is observed as the induction period.

A series of St-g-poly(AM-co-NIPAM-co-SAMPS) (ANS copolymers) with diﬀerent NIPAM and SAMPS molar intake were then synthesized by Cu0_{-mediated LRP with StBr as the}

initiator and copper powder/Me6Tren as the catalyst system

(Table 2). Information for the synthesis of St-g-poly(AM-co-NIPAM) (ANS1000 and ANS2500) is also listed inTable 2 for the clarity of discussion, details about these two copolymers are available in our previous report.12

According to our previous work, the target DP for all the samples was set to 6000 to achieve satisfactory viscosity values for potential applications such as EOR. The copolymers synthesized in the present work were characterized by FT-IR (Figure 2a) and 1_{H-NMR (}_{Figure 2}_{b). The absorption peak}

around 3188 cm−1 in FT-IR spectrums was assigned to the stretch of the N−H bond in the amide group. For the ANS0000, a typical amide group peaks at 1652 cm−1(amide I) and 1610 cm−1(amide II) could also be seen in the spectrum. With the increase of SAMPS and (or) NIPAM intake in the copolymer, the amide I and II peaks gradually shifted to 1628 and 1530 cm−1, respectively.39 The symmetrical and asym-Table 1. Experimental Data of Linear PSAMPS (co)polymer Synthesized by Cu0-Mediated LRP

sample [M]:[L]:[Cu0_]:[L] monomer_ratioa _(SAMPS)ratio _DPc _PDId

entry 1 50:1:0.6:0.6 100:0:0 100 46 1.32 entry 2 500:1:1:0.8 100:0:0 N.A. entry 3 500:1:2:2 100:0:0 100 390 1.47 entry 4 2000:1:2:2 75:0:25 23.5b 1178 1.45

a_{The ratio of AM/NIPAM/SAMPS in feeding solution.} b_The composition of copolymer determined by elemental analysis, data see support information (Table S1).cThe DP determined according to NMR and mass, N.A. means no reaction.dThe PDI determined by GPC.

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https://dx.doi.org/10.1021/acs.iecr.9b06893 Ind. Eng. Chem. Res. 2020, 59, 10847−10856 10849

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metrical stretch of the SO bond in the sulfonate group could also be seen at 1040 and 1180 cm−1, respectively.40 The1

H-NMR spectra of the copolymers are shown in Figure 2b, in which the peak around 1.0 ppm was attributed to the methyl protons in the NIPAM unit while the peak around 1.3 ppm was attributed to the methyl protons in the SAMPS unit. The peak at 3.8 ppm originates from the tertiary carbon protons in the NIPAM amide group while the broad peak in the range of 3.0−3.5 ppm originates from the SAMPS methine carbon (connected with the sulfonic group) protons. The signals in

the range of 1.9−2.3 ppm and 1.2−1.8 ppm (partially overlap with the peak from SAMPS methyl protons) were assigned to the tertiary and secondary carbon protons in the copolymer backbone, respectively.

As shown inTable 2, the mole ratio of SAMPS and NIPAM in the feeding monomer mixture were varied from 0 to 50 and 0 to 25%, respectively, while the overall target DP was set to 6000 for all the copolymer. Elemental analysis, instead of NMR due to the overlapping peaks in the spectra, was used to determine the mole percentage of the SAMPS unit (for all copolymers) and AM unit (for copolymers with NIPAM) in the product according toeqs 1 and2 (see the Materials and Methods section).

The polymerization kinetics were not monitored because of the relatively high rate and corresponding high solution viscosity. However, the controllability of Cu0-mediated LRP on the grafting can be verified, although indirectly, by the PDI (obtained from GPC) of the (co)polymer cleaved from the starch backbone. As shown in Table 2, narrow molecular weight distribution (PDI < 1.6) which suggests a reasonably controlled polymerization is observed after the hydrolyzation of the starch backbone. Similar to the GPC traces shown in other reports,41,42 the (co)polymers cleaved from the starch backbone also display a non-normal distribution (Figure S4). Given the rheology test results shown below, the copolymer with higher hydrodynamic volume (thus higher viscosity) displays larger deviation from normal distribution which might be explained by the residual“deactivation by propagation” as proposed in the literature.43 Accordingly, although the “gel effect” at high conversion constrains the movement of polymer chains and thus results in the inefficient control on molecular Figure 1.Kinetic plot of the PSAMPS homopolymer synthesized by Cu0_{-mediated LRP.}

Table 2. Experimental Data of Starch-Based Grafted Copolymer Synthesized by Cu0-Mediated LRP conversion/%b ratioc

sample monomer ratioa time/min AM NIPAM SAMPS NIPAM SAMPS DPb PDId

ANS0000 100:0:0 12 91.56 5554 1.42 ANS0010 90:0:10 12 96.95 94.80 9.8 5804 1.47 ANS1000 90:10:0 12 80.66 80.66 10.0 4840 1.64 ANS0025 75:0:25 15 90.67 90.19 24.9 5433 1.51 ANS2500 75:25:0 15 92.57 82.95 23.0 5410 e ANS1015 75:10:15 15 89.55 91.96 90.26 10.2 15.1 5394 1.54 ANS2525 50:25:25 20 86.19 95.34 85.26 27.0 24.2 5295 e ANS0050 50:0:50 18 92.25 83.80 47.6 5282 1.53

a_{The ratio of AM/NIPAM/SAMPS: in feeding solution. Overall ratio for the grafting [M]:[I]:[Cu}0_{]:[L] = 6000:1:2:2.}b_{Monomer conversion and} polymer DP determined by elemental analysis (or NMR) and mass.cThe composition of grafted chains determined according to elemental analysis or NMR.dThe PDI of the copolymer after the hydrolyzation of starch backbone.eNot available due to potential high intermolecular association, leading to retention of the polymer inside the GPC column.

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weight, the activation/deactivation cycle can still be main-tained because of the diﬀusion of activation/deactivation species. Moreover, the radical centers can also “reach” the deactivation species by propagation (residual deactivation) which is helpful to lower the PDI.

To investigate the inﬂuence of chain composition on copolymer solution properties, a series of rheology tests were carried out. For a polymer with a given molecular structure, the intrinsic viscosity is an indication of the hydrodynamic volume of polymers.44,45 The intrinsic viscosity can be obtained by extrapolating the plot of ln(η_red) against polymer concentration to c = 0 (see the Materials and Methods section). It should be noted that for linear polyelectrolytes the electrostatic repulsion

expands the macromolecule coil in dilute solution because of intramolecular repulsion but compresses that in higher concentration because of intermolecular repulsion. As a result, higher reduced viscosity can be observed at a lower polymer concentration (known as polyelectrolyte effect). This makes the extrapolation to infinite dilution quite unreliable, so the measurements for linear polyelectrolyte usually are carried out in a salt solution (such as 0.1 M NaCl) to screen out the influence of charges.46,47However, this makes the comparison between samples inconvenient as the salt concentration varies in order to effectively shield different charge density, while the final result is also related to the salt concentration.48 Furthermore, in the case of highly branched polyelectrolytes, Figure 2.FT-IR (a) and1H-NMR (b) spectrum of starch-based graft copolymers.

Figure 3.Reduced viscosity as a function of polymer concentration (Martin equation) (a) and corresponding intrinsic viscosity (b).

Figure 4.Viscosity as a function of shear rate (a, 1.2 wt % copolymer solution) and corresponding relaxation time from Carreau−Yasuda model (b).

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the steric hindrance and strong intramolecular Coulomb repulsion may, to some extent, oﬀset the “polyelectrolyte eﬀect” encountered by linear ones. This makes the measure-ment and comparison of intrinsic viscosity in a salt-free solution possible. Attempts were made with the Martin equation in the present research, and results are shown in Figure 3 (part of the data for ln(ηred) versus concentration

were shown inFigure S5for clarity).

As can be observed inFigure 3b, with the same molar ratio in the copolymer, a SAMPS unit can expand the hydrodynamic volume of the grafted copolymer more significantly than the NIPAM unit because of the Coulomb repulsion forces. The changes in the intrinsic viscosity between grafted AM/SAMPS bi-component copolymers also signify an optimum SAMPS intake (25 mol %) in the composition. Interestingly, compared to the increase of intrinsic viscosity between ANS0010 and ANS1015, a decrease can be observed between ANS0025 and ANS2525, which indicates a negative influence of the NIPAM unit on the hydrodynamic volume of the copolymer. This trend is in agreement with the sequence of the low shear rate viscosity (γ < 10 s−1inFigure 4a) of copolymers which is more related with the hydrodynamic volume and excluded volume.49−51 One may argue that the (co)polymer concen-tration is still too high to show the “polyelectrolyte effect”. However, this effect can be observed in the same concentration range according to one previous report on spherical polyelectrolytes.52Nevertheless, the validity of measuring the intrinsic viscosity of synthesized starch-based branched polyelectrolytes with the Martin equation in salt-free water still needs further investigation.

The influence of chain composition on solution viscosity as a function of shear rate was also evaluated at the same polymer concentration, the result of which is shown inFigure 4a. It is clear that because of the extended molecular chains resulting from the Coulomb repulsion forces, the viscosity of AM/ SAMPS bi-component copolymers is much higher than that of AM/NIPAM bi-component copolymers, especially in the low shear rate region. It is also noticed that copolymers with a higher SAMPS intake (e.g., > 25 mol %, ANS0050) are more shear sensitive at a high shear rate (>10 s−1) than other copolymers. This is attributed to the weak chain entanglement because of the Coulomb repulsion forces. Furthermore, these data were alsofitted with the Carreau−Yasuda model (see the Materials and Methods section) to study the influence of chain composition on the copolymer relaxation time (λ).9,35,36 As indicated inFigure 4b, a higher SAMPS intake results in higher λ values, thus, the lower critical shear thinning rate (1/λ)

should be observed in the flow curve. This is in line with Figure 4a which shows a shift in the onset of shear thinning behavior toward lower shear rate regions. In the case of an AM/NIPAM/SAMPS tri-component copolymer, high NIPAM intake, like its effect on the intrinsic viscosity, reduces the relaxation time which means the copolymer solution is less shear sensitive (comparing the λ of ANS0025, ANS2525 in Figure 4b and correspondingflow curve in Figure 4a). This may be related to the intermolecular association because of the high ratio of NIPAM units.

Besides viscosity, the viscoelastic property also affects the performance of polymers in applications such as improving the sweep efficiency in EOR.53The influence of composition on the viscoelastic properties of the copolymers is shown inFigure 5(for clarity the plot of loss modulus vs frequency is shown in Figure S6). InFigure 5a, in the terminal zone (low frequency region), typical Maxwellflow behavior with G′ proportional to ω2 _{(slope = 2) and G}_{″ directly proportional to the angular}

frequency (ω) (slope = 1) can be observed for samples with low SAMPS intake.54For copolymers with high SAMPS intake (e.g., > 15 mol %, ANS1015), deviation in the slope from the Maxwell model is observed which suggests intermolecular entanglement.49 Similar to the viscosity profile shown in the flow curve inFigure 4a, both the storage modulus (G′) and the loss modulus (G″) of SAMPS-containing copolymers increased more significantly (ω < 20 rad/s) compared with those of AM/NIPAM bi-component copolymers. The same trend can also be observed in the change of solution elastic response as indicated by the phase angles at equal copolymer concen-trations (Figure 5b). All these results, such as the intrinsic viscosity and viscosity versus shear rate curve (vide supra), indicate an optimum SAMPS ratio (25 mol %) in the copolymers as well.

As evidenced by the results shown above, at a high molar ratio the NIPAM intake displays a negative influence on the rheology performance of the SAMPS copolymer in fresh water. In application areas such as EOR, an inorganic electrolyte present in the solution can screen the Coulomb forces which may result in different rheology behavior. Thus, the influence of salt and NIPAM intake on the viscosity of the copolymer was also studied in saline water with different NaCl concentration. As shown in Figure 6, all the SAMPS copolymers display higher viscosity than that of AM/NIPAM bi-component copolymers (optimally, 220 mPa·s, see ref12) when the SAMPS intake is high enough (e.g., > 10 mol %). Comparison between different SAMPS-containing copolymers also indicates 25% is the optimum SAMPS ratio for their saline Figure 5.Storage (G′) (a) and phase angle (b) as a function of frequency at 1.2 wt % polymer concentration.

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resistance in viscosity: while a lower ratio is not enough to eﬀectively break down the hydrogen bond between AM units in saline water (ANS0010), a higher ratio makes the hydrodynamic volume of the copolymers more sensitive to salt (ANS0050). The saline sensitivity of ANS0050 is further proved when comparing its viscosity with that of ANS1015 and ANS0025 when the salinity is higher than 50,000 ppm. In the case of AM/NIPAM/SAMPS tri-component copolymers, the incorporation of the NIPAM unit leads to a lower viscosity

which is related to the intramolecular association of NIPAM units at a high salt concentration because of the collapse of the macromolecule (comparing the viscosity of ANS0025 and ANS2525 inFigure 6).

Besides the influence of NIPAM on the room temperature rheology, the thermo-thickening behavior of the NIPAM copolymer is also of interest for applications such as EOR. According to our previous report, the thermo-thickening behavior is not only affected by the intake of the NIPAM unit, but also by the strong intramolecular hydrogen bond in the waxy starch-based highly branched AM copolymer.12 In the present research, the Coulomb repulsion between SAMPS units breaks down the intramolecular hydrogen bond and also hinders the intermolecular association of NIPAM units in fresh and low salinity (<25,000 ppm) water. As a result, no thermo-thickening behavior is observed at a shear rate of 1 s−1in group A of Figure 7a. With the increasing salinity, charges on the polymer chains are screened effectively which makes the hydrophobic association of NIPAM units possible. This is proved by the thermo-thickening behavior indicated by group B (Figure 7a) and the more obvious increment of viscosity in a higher salinity solution (group C,Figure 7a). At a higher shear rate (γ = 3 s−1), the hydrophobic association can be disrupted, as the result of which thermo-thickening is only observed at really high salinity (more hydrophobic association) as can be seen inFigure 7b. At an even higher shear rate, for example, the average shear rate in porous media (γ = 10 s−1) as reported, no thermo-thickening behavior of AM/NIPAM/ SAMPS tri-component copolymers is observed in Figure 7c,d.55,56 The viscosity−temperature profiles of different copolymers are also shown in the same graph for comparison. As can be observed, SAMPS-containing bi-component Figure 6.Viscosity (γ = 10 s−1) as a function of salinity (unit in kilo

ppm) for starch-based graft copolymers (1.0 wt %).

Figure 7.Viscosity vs temperature of waxy potato starch-based AM/SAMPS/NIPAM copolymers in saline water at diﬀerent shear rates (1.0 wt % copolymer concentration).

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copolymers display better temperature and saline resistance compared with tri-component copolymers. This is more obvious in the case of ANS0050 which has a lower viscosity under room temperature compared with other copolymers but displays higher viscosity than AM/NIPAM/SAMPS tri-component copolymers at high temperature and salinity (e.g., above 40°C and 50,000 ppm NaCl). This may also be related to its higher charge density in comparison with other copolymers. Considering the higher viscosity of ANS2525 relative to that of ANS1015 in fresh water (Figure 6), the intake of a high mole ratio of the NIPAM unit in the copolymer again proved to play a negative role on the saline resistance property of the copolymer in a wide temperature range.

4. CONCLUSIONS

In this work, a water-soluble waxy potato starch macroinitiator was homogeneously synthesized in DMAc. A series of random copolymers of AM, SAMPS, and NIPAM were then grafted from the macroinitiator via aqueous Cu0_{-mediated living}

radical polymerization (Cu0-mediated LRP) at room temper-ature. The mole ratios of SAMPS and NIPAM were varied in the range of 0−50 and 0−25%, respectively, to investigate the influence of chain composition on copolymers’ aqueous rheological properties as well as their thermo-responsive properties. Because of the negatively charged side groups, waxy potato starch-based AM/SAMPS bi-component copoly-mers displayed much higher viscosity than AM/NIPAM bi-component copolymers in saline solution. Comparison of intrinsic viscosity, viscosity profile at different shear rates, and salinity between different copolymers indicated an optimum SAMPS ratio (25 mol %) for the balanced performance of viscosity and salt resistance. The intake of a high ratio of the NIPAM unit undermines the thickening ability of AM/ NIPAM/SAMPS tri-component copolymers) in both fresh and saline water because of hydrophobic association. In a high salinity solution, the hydrodynamic volume of the macro-molecule collapses because of the screening effect of salt on the charged SAMPS units. As the result, thermo-thickening behavior can be observed at low shear rates (γ ≤ 3 s−1) because of hydrophobic association. At a higher shear rate, the thermo-thickening behavior disappears because of the disruption of association.

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ASSOCIATED CONTENT

*

sı Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.iecr.9b06893.

Synthesis of waxy potato StBr and St-g-PAM, illustration for the highly branched structure of amylopectin and its copolymer, FT-IR spectra of StBr with diﬀerent DS,1

H-NMR, 13C-NMR, and gHSQC spectra of StBr (DS = 0.15) in D₂O, elemental analysis result of starch-based graft copolymer, GPC traces of (co)polymers cleaved from a starch backbone, reduced viscosity as a function of polymer concentration (Martin equation), and loss modulus versus frequency of starch-based copolymer (1.2 wt %) (PDF)

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AUTHOR INFORMATION

Corresponding Author

Francesco Picchioni − Engineering and Technology Institute Groningen, University of Groningen, 9747AG Groningen, The Netherlands; orcid.org/0000-0002-8232-2083;

Phone: +31-050-363-4333; Email:f.picchioni@rug.nl

Authors

Yifei Fan − Engineering and Technology Institute Groningen, University of Groningen, 9747AG Groningen, The Netherlands Ranjita K. Bose − Engineering and Technology Institute

Groningen, University of Groningen, 9747AG Groningen, The Netherlands

Complete contact information is available at: https://pubs.acs.org/10.1021/acs.iecr.9b06893

Funding

This work was performed under theﬁnancial support from the China Scholarship Council (CSC).

Notes

The authors declare no competingﬁnancial interest.

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ACKNOWLEDGMENTS

We thank the ﬁnancial support from the China Scholarship Council (CSC) (grant number: 201406380107). We also thank Avebe (Veendam, The Netherlands) for the donation of the waxy potato starch.

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